Preface to First Edition - lib

SYSTEMATIC REVIEWS AND META-ANALYSIS 269Table 15.2:BCG data. Meta-analysis on BCG vaccine with thefollowing data: the number of TBC cases after avaccination with BCG (BCGTB), the total number ofpeople who received BCG (BCG) as well as the numberof TBC cases without vaccination (NoVaccTB)and the total number of people in the study withoutvaccination (NoVacc).Study BCGTB BCGVacc NoVaccTB NoVacc Latitude Year1 4 123 11 139 44 19482 6 306 29 303 55 19493 3 231 11 220 42 19604 62 13598 248 12867 52 19775 33 5069 47 5808 13 19736 180 1541 372 1451 44 19537 8 2545 10 629 19 19738 505 88391 499 88391 13 19809 29 7499 45 7277 27 196810 17 1716 65 1665 42 196111 186 50634 141 27338 18 197412 5 2498 3 2341 33 196913 27 16913 29 17854 33 197615.2 Systematic Reviews and Meta-AnalysisMany individual clinical trials are not large enough to answer the questionswe want to answer as reliably as we would want to answer them. Often trialsare too small for adequate conclusions to be drawn about potentially smalladvantages of particular therapies. Advocacy of large trials is a natural responseto this situation, but it is not always possible to launch very largetrials before therapies become widely accepted or rejected prematurely. Onepossible answer to this problem lies in the classical narrative review of a setof clinical trials with an accompanying informal synthesis of evidence fromthe different studies. It is now generally recognised, however, that such reviewarticles can, unfortunately, be very misleading as a result of both the possiblebiased selection of evidence and the emphasis placed upon it by the reviewerto support his or her personal opinion.An alternative approach that has become increasingly popular in the lastdecade or so is the systematic review which has, essentially, two components:Qualitative: the description of the available trials, in terms of their relevanceand methodological strengths and weaknesses.Quantitative: a means of mathematically combining results from different© 2010 by Taylor and Francis Group, LLC